Miliohm meter , any schematics or good price

[chichisport]

Hello:

I think I need an IR meter to test cells and motor , but the one I see cost 180 euros.Can anyone show me schematics to make one or somewhere to buy it cheaper.The needed acuracy is 0,1 miliohm.

Thank you.

Juan Enrique.

[Comatose]

You can't measure the IR of cells with a multimeter. What you need to do is get a fixed discharge current coming out of them, and then measure the voltage depression. Depending on the cell, 1A, 5A 10A or 50A might be the best value to test at. Even better would be measuring the internal resistance for a variety of data points.

The circuit for this would look a lot like a buck converter running in constant current mode, feeding a large power resistor or a light bulb. You'd then read the delta of voltage between off state and steady state on on the input side. (input goes to the battery)

For a milliohm meter, you'll need a constant current source, then read the voltage out with the rotor stalled.

Either way it looks like you'll be building a big constant current source, then either use a microcontroller or just a multimeter to measure the voltage drop.

[chichisport]

Thanks comatose:
I was thinking the same ,but I´m refering to a direct instrument like robbe sells:
http://shop.lindinger.at/ProductDetail.asp?ProductID=16503&NextPage=%2FProductTree%2Easp%3FCategoryID%3D321%2 6PPP%3D%26SP%3D0

[chichisport]

The manufacturer instructions:
http://data.robbe-online.net/robbe_pdf/P1122_1-8422.pdf

[Mr.RC-CAM]

I understand some folks are using HP milliohm meters, with a series cap, to measure the cell impedance. Apparently the lab grade meters use an AC current, which works well with the DC blocking cap.

I have no idea if this is accurate, at least in a way that would duplicate the manufacturer's results. But, I think it might give a relative cell impedance value that can be used to monitor the cell as it ages.

Has anyone tried this odd method? If so, does it seem to give reliable (and believable) data?

RC-CAM

[chichisport]

Oh, more questions,Is this miliohm meter cheaper than 180$?

Thanks.

[Plantoflap]

Another way to find the internal resistance of your batteries is to buy an RMS digital VOM and a low ohm power resistor and use the resistor in place of the motor. Give it about half throttle or less. Measure the rms voltage across the resistor (before it gets too hot)and divide by ohms to find the current. Then measure RMS AC volts across the battery. The internal resistance of the battery would be RMS volts/RMS current. It is best to use a true rms meter to do the measurements as I have used my oscilloscope to observe the waveform at the battery as the esc switched on/off. It is not a pure rectangular waveform so an averaging volt meter will not be as accurate. I could see the effects caused by the motor noise cap for instance.
RMS DMMs are available for less than $180 at www.Jameco.com

[chichisport]

Thanks Plantoflap,but still trying to find something in one piece , so I go to the shop and see what I´m going to buy(Batteries or cells).

Juan Enrique.

[vintage1]

why not build a Wheatsone Bridge?

A decent DVM a single good pot with a calibrated scale, and a single precision resistor round the 1-10 ohm mark will be all you need.

In fact I have seen wheastone bridges on Ebay at very sensible money.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=294&item=5718865141&rd=1&ssPageName=WDVW

at least you get te switches, the calibrated pot and a decent meter: if it won't do the range you want, a simple change of internal resistor will get that correct.

[chichisport]

Please Vintage 1 ,correct me if I´m wrong:
I made a W bridge , get balanced by the pot , and then conect the wanted to meassure cell in parallel to the adjusting pot so I can measure the inbalance in the meter.
Questions :
A charged cell placed between a resistor is discharged ,so we need a large value resistor or it will make boom (10 ohms for 0.12 Amps)
How precise will be the meassure of 3 to20 miliohm ?
The robbe instructions said to use the four tips of the 2 probes to measure and zero adjust , any tracks of what are they doing ?

A lot of thanks.

[vintage1]

Oh. cant use it to measure cell internal resistnace - sorry. well not easily anyway.

Thought you wanted to measure motor resistance.

easiets way to measure cell impedance is a whattmeter and a load resistor - take some measurements of current and voltage and plot a graph. Internal; resistance is slope of best fit line through teh points.

[chichisport]

I suspect the same Vintage 1.
Maybe someone can help to get a "portable ri meter" like the robbe one.

Thanks.

[picclock]

Hi
easiest way to get accurate results is to use very low value load - say 1 ohm or less and switch it on and off with a low duty cycle say 1%. This way the resister only dissipates 1% of the dc load power. You can then measure the ripple voltage from the cell(s) to derive the internal source impedance.

The load can be switched using a 555 timer and a suitable FET, providing you put a diode accross the 'middle' resister to make the output normally low pulsed high.

The internal resistance of a cell will vary with load current, charge state, and temperature. Additionally it will vary with the length of pulse, and with Li cells, when used after storage the resistance wil start out high, then reduce, then increase again as the cell discharges due to a film which builds up around the cell anode when the cell is not used.

Best Regards

Martin R Dare

[chichisport]

Thanks Martin:
From your post I must think that the robbe meter has any sort of micro to do the math or using different technic?

Juan Enrique.

[picclock]

I'm afraid my german is no good. It appears from the instruction pdf that the device uses the same technique as I described, with the frequency being 1kHz.

If you measure the change in voltage (which can be done with an AC voltmeter) then its simply a matter of multiplying that voltage by the load resistance. So 20mV, 1 Ohm load = 20mOhm resistance. 20mV with 5 ohm load will be 100mOhm internal resistance.

Best Regards

Martin R Dare

[LT]

German company ELV( www.elv.de ) got a meter called RIM 1000 that cost approx 90 EOR or 75 EUR as a kit. Go to their site and do a search for this product for more information.
Additional information can be found in this german manual that also includes the schematics:
http://www.elektromodellflug.de/Uploads/rim2003_V3.pdf
For a short review of the product (in german):
http://www.elektromodellflug.de/rim_1000.htm

Regards
Lars

[chichisport]

These begin to be a portable meter;
It´s a complex instrument , good price. Thank you.

Did you see his home charger with 4 outputs.